Polycarbonate is a well known thermoplastic polymer useful for applications wherein good electrical resistance, distortion temperature under load and impact resistance are valuable properties. There are many known processes for preparing polycarbonate, including inter alia the lime process, the pyridine process, melt polymerization, transesterification, solution process, and interfacial polymerization. Virtually all of these, if not all of these processes, start out initially with a dihydric phenol and a carbonate precursor. As in almost all other reactions there is not 100% yield of the desired high molecular weight aromatic polycarbonate. There are side reactions which bring about product other than the desired polycarbonate. Some of these well known side products are diphenyl carbonate, sodium carbonate, and linear oligomers of the desired polycarbonate. In order to enhance the properties of the polycarbonate it is important to minimize side reaction products present in the polycarbonate. Usually the approach taken is to minimize the side reaction as opposed to removal of the side reaction product because of the additional, sizeable expense involved.
A new and inexpensive method of purifying high molecular weight aromatic polycarbonate has been found. In addition, it has been found that the side reaction product is particularly rich in cyclic polycarbonate oligomers. These oligomers can be isolated and further polymerized to linear high molecular weight polycarbonate per se. A further use of the cyclic oligomers is the addition to or the impregnation with fibrous materials such as glass, carbon or aramid fibers followed by the in situ polymerization of the polycarbonate to form composites having great modulus strength.